The present invention relates to an optical amplifier which forms a part of an optical communication system.
Recently, the optical communication system which makes use of an optical fiber has increasingly been developed because of its low induced noise and it's ability of long-distance transmission as compared with the system which makes use of a metallic transmission line. In addition, an amplifier is arranged in the middle of the transmission line for the achievement of more longer distance transmission. Conventionally, a signal laser beam has once been converted into an electric signal to electrically amplify the laser beam and then the electric signal has been used for generating a laser beam to thus give an amplified laser beam, but it is a recent tendency to use an optical amplifier for the purpose of preventing such reduction in efficiency due to the amplification of an optical signal through conversion thereof into an electric signal.
Among optical amplifiers, it is particularly effective to use an optical fiber for amplification if it is arranged in the middle of a transmission line for the optical communication. The optical fiber for amplification is a fiber comprising a core portion to which a rare earth element such as erbium is added. A signal laser beam which is guided to and passes through the optical fiber is directly amplified through the stimulated emission of the rare earth element which receives an energy emitted from a laser for excitation.
The use of an amplifying system as shown in FIG. 7 is required for inserting an amplification optical fiber of this type into the transmission line of an optical communication system. As seen from FIG. 7, an optical fiber 1 for transmission is connected to a wavelength division multiplexer 6 at the light-incident side thereof through an optical isolator 2. Moreover, a laser 3 for excitation is also connected to another position on the light incident side of the wavelength division multiplexer 6 through an optical isolator 4. On the other hand, an optical fiber 8 for amplification is connected to the wavelength division multiplexer 6 at its light-outgoing side. In this respect, another end face of the wavelength division multiplexer 6 at the light-outgoing side is subjected to an anti-reflection treatment 7. The optical fiber 8 for amplification is connected to an optical isolator 9 at its light-outgoing side and the optical isolator 9 is connected to an optical fiber 11 for transmission, at its light-outgoing side, through a band-pass filter 10.
Each of the optical fibers 1 and 11 for transmission is a single mode fiber capable of transmitting a signal laser beam falling within a band having a wave length .lambda. of 1.55 .mu.m and only a part of the overall length thereof is depicted on FIG. 4. The optical isolator 2 comprises a polarizer, a Faraday rotator and analyzer and serves to prevent the light incident upon the optical fiber 1 for transmission from being returned to the optical fiber 1 through surface-reflection by the light-incident face of the wavelength division multiplexer 6. Each of the optical isolators 4 and 9 has a structure identical to that of the isolator 2. The laser 3 for excitation serves as a source for oscillating an excitation laser beam having a wave length .lambda. of 1.48 .mu.m. The amplifying optical fiber 8 is an optical fiber obtained by adding erbium to the core portion thereof. The wavelength division multiplexer 6 can be prepared by heating and fusing two optical fibers 61 and 62 which are superimposed to one another while drawing the heated and fused fibers. These fibers 61 and 62 each comprises a core portion 61a or 62a and a clad portion 61b or 62b. The fiber 61 is a currently used single mode fiber for transmission like the optical fibers 1 and 11 for transmission, while erbium is added to the core portion of the fiber 62, as in the case of the optical fiber 8 for amplification. The band-pass filter 10 is a selective transmission type optical filter capable of transmitting only the band peak having a wave length .lambda. of 1.55 .mu.m.
In the amplification system shown in FIG. 7, a signal laser beam (wave length .lambda.=1.55 .mu.m) transmitted through the optical fiber 1 for transmission is incident upon the wavelength division multiplexer 6 through the optical isolator 2. Moreover, a laser beam (wave length .lambda.=1.48 .mu.m) for excitation oscillated from the excitation laser 3 is incident upon the wavelength division multiplexer 6 in which the laser beam is multiplexed with the signal laser beam and the multiplexed laser beam is then incident upon the optical fiber 8 for amplification. The signal laser beam is amplified through the stimulated emission of the erbium which is excited by the laser beam for excitation during the passage of the signal laser beam and the laser beam for excitation through the optical fiber 8 for amplification. The signal laser beam thus amplified is incident upon the band-pass filter 10 through the optical isolator 9, the band-pass filter 10 cuts off light rays of unnecessary wave lengths (mainly comprising those having a wave length .lambda. of 1.48 .mu.m) to thus allow the passage of only the signal laser beam (having wave length .lambda. of 1.55 .mu.m) and the amplified signal laser beam is further transmitted through the optical fiber 11 for transmission.
The aforementioned amplification system which makes use of the optical fiber for amplification has a markedly improved efficiency as compared with the conventional system in which a signal laser beam is once electrically amplified through the photoelectric conversion and then the resulting electric signal serves to oscillate an amplified laser beam, but it is needed to fuse the end faces of optical fibers in order to eliminate any loss at joined portions each formed between neighboring two optical elements. In FIG. 7, these joined portions each formed between neighboring two elements fused together are denoted by symbols a to g. Even if the joined portions are fused, the propagation loss is inevitably caused at such joined portions and accordingly, it has become an important technical subject in this field to reduce the propagation loss as low as possible from the viewpoint of the overall degree of amplification. Japanese Patent Provisional Publication No. 4-67130 discloses a technique for approximately coinciding the mode field of a fiber with that of another fiber to be joined by heating the fused and joined portion to thus diffuse the dopant present in the core region of the fibers. However, this technique is still incomplete and a finite degree of propagation loss is still observed in such a fused portion.